RIght, we observe an evolving pattern in fossils for instance, we observe similar patterns when looking at genetic 'distances' for instance. The data supporting that 'evolution' happened is quite overwhelming. The remaining question is, how does one explain the observations. Observations from the past are as good as any btw.
http://www.talkorigins.org/faqs/evolution-fact.html has some information which can help here.
It is time for students of the evolutionary process, especially those who have been misquoted and used by the creationists, to state clearly that evolution is a FACT, not theory, and that what is at issue within biology are questions of details of the process and the relative importance of different mechanisms of evolution. It is a FACT that the earth with liquid water, is more than 3.6 billion years old. It is a FACT that cellular life has been around for at least half of that period and that organized multicellular life is at least 800 million years old. It is a FACT that major life forms not on earth were not at all represented in the past. There were no birds or mammals 250 million years ago. It is a FACT that major life forms of the past are no longer living. There used to be dinosaurs and Pithecanthropus, and there are none now. It is a FACT that all living forms come from previous living forms. Therefore, all present forms of life arose from ancestral form
s that were different. Birds arose from nonbirds and humans from nonhumans. No person who pretends to any understanding of the natural world can deny these facts any more than she or he can deny that the earth is round, rotates on its axis, and revolves around the sun.
The controversies about evolution lie in the realm of the relative importance of various forces in molding evolution.
- R. C. Lewontin "Evolution/Creation Debate: A Time for Truth" Bioscience 31, 559 (1981) reprinted in Evolution versus Creationism, op cit.
Or since the Fruit-fly was brought up (I apologize for the length)
5.3 The Fruit Fly Literature
5.3.1 Drosophila paulistorum Dobzhansky and Pavlovsky (1971) reported a speciation event that occurred in a laboratory culture of
Drosophila paulistorum sometime between 1958 and 1963. The culture was descended from a single inseminated female that was
captured in the Llanos of Colombia. In 1958 this strain produced fertile hybrids when crossed with conspecifics of different strains
from Orinocan. From 1963 onward crosses with Orinocan strains produced only sterile males. Initially no assortative mating or
behavioral isolation was seen between the Llanos strain and the Orinocan strains. Later on Dobzhansky produced assortative
mating (Dobzhansky 1972).
5.3.2 Disruptive Selection on Drosophila melanogaster Thoday and Gibson (1962) established a population of Drosophila melanogaster
from four gravid females. They applied selection on this population for flies with the highest and lowest numbers of sternoplural
chaetae (hairs). In each generation, eight flies with high numbers of chaetae were allowed to interbreed and eight flies with low
numbers of chaetae were allowed to interbreed. Periodically they performed mate choice experiments on the two lines. They found
that they had produced a high degree of positive assortative mating between the two groups. In the decade or so following this,
eighteen labs attempted unsuccessfully to reproduce these results. References are given in Thoday and Gibson 1970.
5.3.3 Selection on Courtship Behavior in Drosophila melanogaster Crossley (1974) was able to produce changes in mating behavior in
two mutant strains of D. melanogaster. Four treatments were used. In each treatment, 55 virgin males and 55 virgin females of both
ebony body mutant flies and vestigial wing mutant flies (220 flies total) were put into a jar and allowed to mate for 20 hours. The
females were collected and each was put into a separate vial. The phenotypes of the offspring were recorded. Wild type offspring
were hybrids between the mutants. In two of the four treatments, mating was carried out in the light. In one of these treatments all
hybrid offspring were destroyed. This was repeated for 40 generations. Mating was carried out in the dark in the other two
treatments. Again, in one of these all hybrids were destroyed. This was repeated for 49 generations. Crossley ran mate choice tests
and observed mating behavior. Positive assortative mating was found in the treatment which had mated in the light and had been
subject to strong selection against hybridization. The basis of this was changes in the courtship behaviors of both sexes. Similar
experiments, without observation of mating behavior, were performed by Knight, et al. (1956).
5.3.4 Sexual Isolation as a Byproduct of Adaptation to Environmental Conditions in Drosophila melanogaster Kilias, et al. (1980) exposed D.
melanogaster populations to different temperature and humidity regimes for several years. They performed mating tests to check for
reproductive isolation. They found some sterility in crosses among populations raised under different conditions. They also showed
some positive assortative mating. These things were not observed in populations which were separated but raised under the same
conditions. They concluded that sexual isolation was produced as a byproduct of selection.
5.3.5 Sympatric Speciation in Drosophila melanogaster In a series of papers (Rice 1985, Rice and Salt 1988 and Rice and Salt 1990)
Rice and Salt presented experimental evidence for the possibility of sympatric speciation. They started from the premise that
whenever organisms sort themselves into the environment first and then mate locally, individuals with the same habitat preferences
will necessarily mate assortatively. They established a stock population of D. melanogaster with flies collected in an orchard near
Davis, California. Pupae from the culture were placed into a habitat maze. Newly emerged flies had to negotiate the maze to find
food. The maze simulated several environmental gradients simultaneously. The flies had to make three choices of which way to go.
The first was between light and dark (phototaxis). The second was between up and down (geotaxis). The last was between the
scent of acetaldehyde and the scent of ethanol (chemotaxis). This divided the flies among eight habitats. The flies were further
divided by the time of day of emergence. In total the flies were divided among 24 spatio-temporal habitats.
They next cultured two strains of flies that had chosen opposite habitats. One strain emerged early, flew upward and was attracted
to dark and acetaldehyde. The other emerged late, flew downward and was attracted to light and ethanol. Pupae from these two
strains were placed together in the maze. They were allowed to mate at the food site and were collected. Eye color differences
between the strains allowed Rice and Salt to distinguish between the two strains. A selective penalty was imposed on flies that
switched habitats. Females that switched habitats were destroyed. None of their gametes passed into the next generation. Males
that switched habitats received no penalty. After 25 generations of this mating tests showed reproductive isolation between the two
strains. Habitat specialization was also produced.
They next repeated the experiment without the penalty against habitat switching. The result was the same -- reproductive isolation
was produced. They argued that a switching penalty is not necessary to produce reproductive isolation. Their results, they stated,
show the possibility of sympatric speciation.
5.3.6 Isolation Produced as an Incidental Effect of Selection on several Drosophila species In a series of experiments, del Solar (1966)
derived positively and negatively geotactic and phototactic strains of D. pseudoobscura from the same population by running the
flies through mazes. Flies from different strains were then introduced into mating chambers (10 males and 10 females from each
strain). Matings were recorded. Statistically significant positive assortative mating was found.
In a separate series of experiments Dodd (1989) raised eight populations derived from a single population of D. Pseudoobscura on
stressful media. Four populations were raised on a starch based medium, the other four were raised on a maltose based medium.
The fly populations in both treatments took several months to get established, implying that they were under strong selection. Dodd
found some evidence of genetic divergence between flies in the two treatments. He performed mate choice tests among
experimental populations. He found statistically significant assortative mating between populations raised on different media, but no
assortative mating among populations raised within the same medium regime. He argued that since there was no direct selection for
reproductive isolation, the behavioral isolation results from a pleiotropic by-product to adaptation to the two media. Schluter and
Nagel (1995) have argued that these results provide experimental support for the hypothesis of parallel speciation.
Less dramatic results were obtained by growing D. willistoni on media of different pH levels (de Oliveira and Cordeiro 1980). Mate
choice tests after 26, 32, 52 and 69 generations of growth showed statistically significant assortative mating between some
populations grown in different pH treatments. This ethological isolation did not always persist over time. They also found that some
crosses made after 106 and 122 generations showed significant hybrid inferiority, but only when grown in acid medium.
5.3.7 Selection for Reinforcement in Drosophila melanogaster Some proposed models of speciation rely on a process called
reinforcement to complete the speciation process. Reinforcement occurs when to partially isolated allopatric populations come into
contact. Lower relative fitness of hybrids between the two populations results in increased selection for isolating mechanisms. I
should note that a recent review (Rice and Hostert 1993) argues that there is little experimental evidence to support reinforcement
models. Two experiments in which the authors argue that their results provide support are discussed below.
Ehrman (1971) established strains of wild-type and mutant (black body) D. melanogaster. These flies were derived from compound
autosome strains such that heterotypic matings would produce no progeny. The two strains were reared together in common fly
cages. After two years, the isolation index generated from mate choice experiments had increased from 0.04 to 0.43, indicating the
appearance of considerable assortative mating. After four years this index had risen to 0.64 (Ehrman 1973).
Along the same lines, Koopman (1950) was able to increase the degree of reproductive isolation between two partially isolated
species, D. pseudoobscura and D. persimilis.
5.3.8 Tests of the Founder-flush Speciation Hypothesis Using Drosophila The founder-flush (a.k.a. flush-crash) hypothesis posits that
genetic drift and founder effects play a major role in speciation (Powell 1978). During a founder-flush cycle a new habitat is
colonized by a small number of individuals (e.g. one inseminated female). The population rapidly expands (the flush phase). This is
followed by the population crashing. During this crash period the population experiences strong genetic drift. The population
undergoes another rapid expansion followed by another crash. This cycle repeats several times. Reproductive isolation is produced
as a byproduct of genetic drift.
Dodd and Powell (1985) tested this hypothesis using D. pseudoobscura. A large, heterogeneous population was allowed to grow
rapidly in a very large population cage. Twelve experimental populations were derived from this population from single pair matings.
These populations were allowed to flush. Fourteen months later, mating tests were performed among the twelve populations. No
postmating isolation was seen. One cross showed strong behavioral isolation. The populations underwent three more flush-crash
cycles. Forty-four months after the start of the experiment (and fifteen months after the last flush) the populations were again tested.
Once again, no postmating isolation was seen. Three populations showed behavioral isolation in the form of positive assortative
mating. Later tests between 1980 and 1984 showed that the isolation persisted, though it was weaker in some cases.
Galina, et al. (1993) performed similar experiments with D. pseudoobscura. Mating tests between populations that underwent
flush-crash cycles and their ancestral populations showed 8 cases of positive assortative mating out of 118 crosses. They also
showed 5 cases of negative assortative mating (i.e. the flies preferred to mate with flies of the other strain). Tests among the
founder-flush populations showed 36 cases of positive assortative mating out of 370 crosses. These tests also found 4 cases of
negative assortative mating. Most of these mating preferences did not persist over time. Galina, et al. concluded that the
founder-flush protocol yields reproductive isolation only as a rare and erratic event.
Ahearn (1980) applied the founder-flush protocol to D. silvestris. Flies from a line of this species underwent several flush-crash
cycles. They were tested in mate choice experiments against flies from a continuously large population. Female flies from both
strains preferred to mate with males from the large population. Females from the large population would not mate with males from
the founder flush population. An asymmetric reproductive isolation was produced.
In a three year experiment, Ringo, et al. (1985) compared the effects of a founder-flush protocol to the effects of selection on
various traits. A large population of D. simulans was created from flies from 69 wild caught stocks from several locations.
Founder-flush lines and selection lines were derived from this population. The founder-flush lines went through six flush-crash
cycles. The selection lines experienced equal intensities of selection for various traits. Mating test were performed between strains
within a treatment and between treatment strains and the source population. Crosses were also checked for postmating isolation. In
the selection lines, 10 out of 216 crosses showed positive assortative mating (2 crosses showed negative assortative mating). They
also found that 25 out of 216 crosses showed postmating isolation. Of these, 9 cases involved crosses with the source population.
In the founder-flush lines 12 out of 216 crosses showed positive assortative mating (3 crosses showed negative assortative mating).
Postmating isolation was found in 15 out of 216 crosses, 11 involving the source population. They concluded that only weak
isolation was found and that there was little difference between the effects of natural selection and the effects of genetic drift.